1. Field of Invention
The object of the present invention is a method for measuring the moisture content or dry-matter content of a high-loss material or of a low-loss material having a moisture content over 50%. The invention also relates to an apparatus for carrying out the method.
2. Description of the Prior Art
In the process industry, a method is needed for measuirng the moisture content or dry-matter content of materials, a method which operates on a real-time basis, i.e. yields the desired values directly. The values obtained can be used for process control, for example.
Previously, the moisture content of materials has been measured by using conductivity measuring or capacitive measuring, in which the change in the conductivity of a material or in its capacitance is measured as a function of the moisture content. These methods have a disadvantage in their unsuitability for high moisture contents and for materials which contain varying quantities of different ions.
Infrared measuring has also been used, but this method is mainly applicable to solids and gases and not to liquids which form a reflecting mirror surface. Furthermore, infrared meters are sensitive to soiling, as are optical devices in general.
In neutron scatter methods the meters measure the quantity of hydrogen per volume unit. The disadvantage of this method is that it is not applicable to materials which contain varying amounts of other hydrogen-containing substances than water. Another problem is that it requires a large amount of the material to be measured. The method is therefore difficult to apply in a process pipe.
Previously known are also various microwave methods which are based either on the high dielectric constant of water, the so-called reflection or resonance methods, or on its high loss factor, i.e. attenuation methods. One example of the disadvantages of the microwave methods is the unsuitability of the attenuation and resonance methods for measuring the moisture content of high-loss materials. The measuring of high moisture contents is also difficult. The same applies also to the reflection method. The reflection method also involves interference problems in practical measuring structures. An attenuation method can be based either on attenuation measuring or on through-travel attenuation measuring of a surface wave.